Actual operating hours change the recurring side of LED retrofit costs, which is the annual kWh and energy charges each lighting system uses. They do not change the one-time installed quote by themselves. More hours raise the modeled energy cost of both the existing and the proposed system, so your estimate is only as good as the hours you put into it.
If your areas run different shifts, split the fixtures into groups that share a schedule. Calculate each group on its own, then add the results. A single facility-wide runtime hides which areas actually drive the cost.
Estimate Annual Lighting Costs With Actual Operating Hours
For each fixture group, multiply fixture count by documented input watts and annual hours, divide by 1,000 to get kWh, and then apply the energy rate. The DOE-hosted commercial lighting evaluation protocol uses the same inputs for baseline and energy-efficient systems: fixture wattage, fixture quantity, and annual hours of use, organized by usage group. Run the calculation below once for the existing system and once for the proposed LED system.
- Collect four inputs for each group and system. You need the fixture count, the system input watts per fixture, the annual operating hours, and the energy rate in dollars per kWh. Take existing-system watts from fixture labels or manufacturer documentation. Take proposed-system watts from the spec sheet for the exact configuration you plan to install, whether you are pricing linear high bay fixtures or other options from our commercial LED lighting range.
- Calculate annual kWh for the group. Annual kWh = fixture count × input watts per fixture × annual operating hours ÷ 1,000. This is the same math as estimating kWh from wattage and runtime for a single space, repeated for each group.
- Apply the group's energy rate. Annual energy charge = group kWh × applicable $/kWh rate. If two groups fall under different rate periods, keep them on separate lines.
- Add up the groups. Sum the existing groups to get one existing-system total. Do the same for the proposed system.
- Compare the two totals. Annual energy-cost difference = existing-system energy cost − proposed-system energy cost.
The hours input scales the result directly. If count, watts, and rate stay the same, doubling a group's annual hours doubles its modeled kWh and energy charge. That is why a 24/7 area and a single-shift area need their own lines.
Give each system its own runtime as well as its own wattage. If the retrofit adds occupancy sensors, daylight controls, or new schedules, the proposed hours can differ from the existing hours. Carrying the old runtime into the new design would misstate the comparison.
Build Annual Hours From Each Fixture Group's Real Schedule
A group's annual hours equal the time its lights actually run in each schedule period, added across the year. The protocol defines a usage group as fixtures sharing operating hours and schedules, and it usually treats baseline and proposed hours as unchanged unless new controls are installed. Use a worksheet like this to keep each group's basis and confidence visible.
| Fixture group | Schedule periods and annual-hour basis | Input status |
|---|---|---|
| Area on one fixed shift | Operating days × lit hours per day | Estimated from posted schedule |
| Area on multiple shifts | Each shift's lit hours, added per day | Observed if control logs exist |
| Area with a seasonal calendar | Hours per season, summed for the year | Estimated until records confirm |
| Intermittent-use area | Logged or sampled on-time | Open until runtime is measured |
| Area getting new controls | Separate proposed-hours value | Estimated for proposed system |
For each period, multiply the number of operating days by the hours the lights are on each day. Then add the period totals to get the group's annual value. A warehouse with a busy season and a slow season, for example, would get two period lines before you combine them.
Mark where each number came from. Lighting control records or dedicated submetered interval data that isolate the lighting load show how long fixtures actually run, so treat those values as observed. Whole-building interval data only reflects overall facility demand unless lighting is monitored separately. A value built only from a posted schedule is an assumption, because building-open hours and fixture runtime can differ when lights stay on for cleaning or turn off in empty zones.
If you are starting from a walkthrough, a fixture schedule from site walk gives each group a traceable row with confirmed, estimated, or open status. When the proposed design uses occupancy sensing, checking high-bay sensor zone size helps you judge whether a zone will really turn off, which shapes the proposed-hours assumption.
Apply the Facility's Utility Rate and Bill Structure
Use the facility's own bills and the tariff that applies in its U.S. service area, not a national average rate. DOE FEMP guidance on utility rates separates energy, demand, and fixed charges, and only one of them comes straight from the kWh calculation above. Find the rate name and each charge on a recent bill, then sort them like this:
- Energy charges belong in the estimate. They are based on kWh consumed, and the cost can vary by time of use and by season. If your tariff changes the rate by time or season, split each group's hours into those rate periods and apply the matching rate to each part.
- Demand charges need separate treatment. They are based on maximum demand in kW during a period, typically each month. Estimate a demand effect only when the tariff and your peak-demand data support it. Interval data, which can show hourly or 15-minute use and monthly peaks, is the input to look for.
- Fixed charges stay outside the calculation. They are a set cost billed monthly, so they do not fall just because lighting uses fewer kWh.
This sorting also tells you how to read the result. A lower lighting kWh total reduces the energy-charge portion of the bill. It does not cut the total utility bill by the same percentage.
Keep Annual Energy Costs Separate From Installed Project Cost
Your annual energy-cost difference is a recurring operating estimate. It is not the installed retrofit price, and it is not a payback result. Label it that way in any budget so nobody reads it as the cost of the project.
The installed price has to come from a current project quote. Fixture wattage, operating hours, and projected energy savings cannot tell you what equipment, labor, or design will cost at your site. If you plan a broader ownership-cost or payback assessment, list each additional input you intend to include and add it only once it is verified for your project. Rebates, maintenance changes, and other benefits fit that rule too: leave them out until you have program terms or site records that support them. For a sense of how energy fits beside fixture, maintenance, and rebate lines, our high bay total cost comparison lays those categories out side by side.
Your next step is practical. Fill in the group worksheet, run the energy calculation for both systems, and then set the result next to the quote when it arrives.
FAQs
How should holidays and planned shutdowns be handled?
Remove those days from the operating-day count for the affected period before you multiply by daily lit hours. If some lights stay on during a shutdown for security or maintenance, give those fixtures their own line with the reduced hours instead of dropping them to zero.
What makes a schedule-based runtime more trustworthy?
Compare it against any observed records you have, such as lighting control logs or utility interval data, for at least the groups that carry the most hours. If the observed runtime differs from the posted schedule, use the observed value and note the change in the worksheet.
Can the same worksheet support a multi-year cost model?
Yes. Use the annual energy line as one input and keep the same groups, watts, and hours behind it. Update the line whenever the tariff, the schedule, or the control strategy changes, because each one changes the annual result.
Do I need new hours if I only swap fixtures and keep the same switches?
Usually not. When the retrofit adds no new controls and the schedule stays the same, the existing and proposed systems can share one annual-hours value per group. In that case, input watts are the only input that differs between the two calculations.
